1. Field of the Invention
This disclosure is related to the field of pipe reduction systems, specifically to pipe reduction systems which use a rigid die to compress a plastic pipe liner for insertion into another pipe via a pulling force exerted by a towing head.
2. Description of the Related Art
Over time, the underground pipelines utilized for the transport of fluids or gases or other elements can become damaged, worn or corroded from use. In the past, the methodologies utilized for rehabilitating these underground or underwater pipelines were costly, labor intensive, and severely disruptive to the surrounding environment and communities.
Today, one of the primary methods and systems utilized in the prior art for rehabilitating existing pipeline systems and networks and to avert these problems is to line an existing pipeline with an extremely tight fitting polyethylene (PE) liner. In such a process, the liner has an outside diameter that is slightly larger than the inside diameter of the pipe being lined, sometimes called a host pipe. Because of the difference in diameter between the liner and the host pipe, the liner is pulled through a die to reduce its diameter before it enters the host pipe.
Generally, the liner is pulled through the die after sections of the liner are butt fused together to form a continuous string. The die temporarily reduces the diameter of the liner. This reduction allows the liner to be easily pulled through the outer existing pipe system. The die used in the prior art systems generally has an entry, a throat and an exit, with the entry decreasing in diameter towards the throat and increasing in diameter away from the throat. Thus, the liner has a maximum diameter before the die, a minimum diameter in the die, and an intermediate diameter after the die. In some embodiments, a heating element is used to apply heat to the liner prior to liner being reduced in the die, the heating element being used to facilitate the reduction of the liner. This is, however, generally less preferred.
The tension given to the liner by the die is generally maintained by a pulling element until the liner is correctly located within the installed pipeline. Commonly, the liner is pulled through the die and the existing pipe system by a winch or towing head. Generally, the force of pulling rendered by the winch or towing head is half the yield strength of the liner or less. It is not uncommon for the forces exerted on the die and winch or pulling head to be very large, often exceeding 100 tons.
Since the liner retains a memory of its original shape and size, it will begin to return to its original shape and diameter when the pulling force is disconnected. After the pulling force is disconnected, the liner relaxes and presses tightly against the inside of the existing pipeline to which it was applied, eliminating any annular space. FIG. 1 depicts the portion of the prior art process in which a new liner (101) is pulled through a reducing die (103) (thereby reducing its diameter) and into the existing pipeline (105) (at its reduced diameter) by a towing head (107) or winch.
Although the prior art process held numerous benefits for the industry, including reducing disruption, creating a strong new pipe, jointless construction, improved flow, and cost savings, the process also has numerous deficiencies in terms of costs, safety and efficiency.
For example, due to the large force vectors exerted, in the prior art system massive ground anchors have to be utilized for both the die and the pulling head in order to withstand these forces. These anchoring systems can be cumbersome, costly, not readily transportable and inefficient.
Another problem with the currently utilized methodology arises from the use of a single reducing die mechanism. Fully reducing the die in a single step often results in extreme point friction on the liner in addition to strain on the liner and joints. This strain and friction often results in mechanical failure of the liner both pre- and post-insertion.
Further, the current systems are generally performed at the level of the pipe. Stated differently, chambers at the level of the pipe (below ground or water) are excavated at each end of the existing pipeline that will be lined. The die of the system is placed within the excavated chamber at the front of the existing pipeline that will be lined. These chambers are costly and time intensive to build. In addition, the excavation involved in creating these chambers can be disruptive to both the environment and the community. Further, because the die is placed within the chamber in these systems, there is often not much space between the reducing die and the existing pipeline, as demonstrated in prior art FIG. 1.
This is problematic for a number of reasons. Mainly, when the tension in the system is released, the inserted liner, in returning to its original diameter, shortens in length. With the die located in such close proximity to the existing pipeline, the end of the liner often gets unintentionally sucked into the existing pipeline, resulting in an incomplete lining situation. This post-release creep can present other problems. After the tension in the system is released, it is not uncommon for the inserted liner to creep or shrink more than expected. Generally, this gradual creep continues for a significant period of time after the insertion and release of the liner. This continued moving and pulling of the inserted liner is problematic because it results in a misformed liner that is susceptible to potential leaks and can pull the liner out of attached fittings.
While longitudinal reversion is expected after tension is released, it is suspected that the liner continues to undergo reversion or creeping for an indefinite period of time, including after the project is completed and the liner is no longer monitored. When a project is completed, however, any surplus liner extending beyond the host pipe is generally trimmed. If the liner continues to revert or creep, the cut end of the liner is at risk of retreating into the host pipe, defeating the purpose of threading a pipe liner in the first place, and rendering it very difficult and expensive to retrieve or augment the liner with additional lengths of liner. Further, fittings are often attached to the end of the liner. If the liner retreats into the host pipe before these fittings are installed, it may become impossible to maintain them. Even if the fittings are installed before such retreat, as the liner continues to creep, these fittings may be exposed to excessive forces jeopardizing the integrity of the joint.
A reduced diameter liner experiences high levels of longitudinal tension resulting in longitudinal stretching. It has been observed that even a relatively short length of pipe may stretch by five to ten feet when exposed to the tensions involved in pipe reduction systems. When the liner is in place and the tension is released, the liner will begin to revert to its shape and size prior to the reduction. Over the course of about a 24 hour period, it has been observed that a liner generally reverts about 80-90% to its initial size and shape, and this reverting process includes length reduction.